Surface Lattice Constant Research Articles

High‐Efficiency InGaN Red Mini‐LEDs on Sapphire Toward Full‐Color Nitride Displays: Effect of Strain Modulation

AbstractInGaN red light emitting diode (LED) is one of the crucial bottlenecks that must be broken through to realize high‐resolution full‐color mini/micro‐LED displays. The efficiency of InGaN LEDs drops rapidly as the emission spectra go from blue/green to red range due to the poor quality of high‐indium‐content InGaN materials. Here, high‐performance InGaN red LEDs on sapphire grown by metal–organic chemical vapor deposition through strain modulation are reported. A composite buffer layer is proposed to increase the surface lattice constant of GaN and hence successfully enhances the indium incorporation efficiency of the following InGaN active layers. Consequently, a high‐efficiency InGaN red mini‐LED chip (mesa area: 100 × 200 µm2) with a peak wavelength of 629 nm and an external quantum efficiency of 7.4% is realized. Finally, a full‐color nitride mini‐LED display panel with 74.1% coverage of Rec.2020 color gamut by using the InGaN red mini‐LED chips is fabricated. The study signifies the great potentials of full‐nitrides high‐resolution full‐color mini/micro‐LED displays.

Electronic band structure of Bi-intercalate layers in graphene and SiC(0001)

A potential way to tune the electronic band structure of graphene is to intercalate foreign atoms in the interface between graphene and substrate. However, such an intercalate layer covered by graphene is difficult to directly study with microscopic probes, and relatively little is known about its crystal and electronic structures. In this work, we study epitaxial graphene on SiC(0001) intercalated by bismuth atoms by means of angle-resolved photoemission spectroscopy. We reveal the electronic band structure of Bi-intercalate layers, from which we could identify two distinct phases, one is metallic and the other is insulating. The metallic phase composed of closely packed Bi atoms shows nearly free electron bands that are repeated to follow the period of SiC(0001)-(1 $$\times $$ 1). The lower coverage insulating phase shows characteristic flat bands in the period of SiC(0001)-( $$\sqrt{3} {\times } \sqrt{3}$$ )R30 $$^\circ $$ with respect to the surface lattice constant of SiC(0001). Even though there exists such two distinct phases in the Bi intercalate layer, the doping level of graphene is found to vary rather continuously with the coverage of Bi. Based on the observed band structures and Bi 5d core-level spectra, we suggest a structural model for the metallic and insulating phases of Bi-terminated SiC(0001).

Determining Surface Terminations and Chirality of Noncentrosymmetric FeGe Thin Films via Scanning Tunneling Microscopy.

Scanning tunneling microscopy was used to study the surfaces of 20-100 nm thick FeGe films grown by molecular beam epitaxy. An average surface lattice constant of ∼6.8 Å, in agreement with the bulk value, was observed via scanning tunneling microscopy, low energy electron diffraction, and reflection high energy electron diffraction. Each of the four possible chemical terminations in the FeGe films were identified by comparing atomic-resolution images, showing distinct contrast with simulations from density functional theory calculations. A detailed study of the atomic layering order and registry across step edges allows us to uniquely determine the grain orientation and chirality in these noncentrosymmetric films.

Experimental determination of surface thermal expansion and electron–phonon coupling constant of 1T-PtTe2

We report an extensive structural characterization of the surface of 1T-PtTe2 crystals by means of helium atom scattering (HAS) and ultra-high resolution scanning tunnelling microscopy (STM). HAS reveals that this bi-dimensional surface presents a large structural corrugation (∼) in direction. The surface lattice constant of PtTe2 derived from the positions of the helium diffraction peaks up to fifth order was found to be , in agreement with STM and x-ray diffraction measurements. This value remains unchanged in the temperature range between 90 and 580 K. These findings suggest that 1T-PtTe2 behaves similarly to graphene on a weakly-interacting substrate, for which the surface thermal expansion coefficient is zero within experimental error. The electron–phonon coupling constant has been determined by recording the thermal attenuation of the elastic diffraction peaks of PtTe2. Based on a recently developed quantum theoretical method adapted to layered degenerate semiconductors we find that lies between 0.38–0.42.

Tuning the Electronic Spin State of Catalysts by Strain Control for Highly Efficient Water Electrolysis

AbstractThe electronic configuration is crucial in governing the binding strength of intermediates with catalysts, yet it is still challenging to control the catalysts' surface electronic spin state. Here, it is demonstrated that through surface metal–organic framework transformation followed by acid etching, the electronic spin state of surface Co3+ ions on spinel Co3O4 can be transformed from t2g6 to the high electronic spin state of t2g4eg2 by expanding the surface lattice constant, which significantly enhances the overlap of the eg orbital of cobalt with the oxygen adsorbates, and greatly improves the intermediates adsorption and thus the oxygen evolution reaction activity. The high electronic spin rich Co3O4 electrode exhibits an anodic current density of 10 mA cm−2 at an overpotential of 280 mV. The finding offers a rational design strategy to manipulate the electronic spin state of catalyst and the hybridization of molecular orbitals in water electrolysis.

Strain engineering of H/transition metal systems

The effects of both compressive and tensile surface strain on the hydrogenated low-index faces of Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt and Au has been investigated using density functional theory (DFT). Changes in the preferred H binding site have been observed on the Pd, Ir and Pt surfaces when the surface lattice constant is strained by up to 2%, and on Fe, Rh, Ag and Os surfaces for larger strains of up to 5%. A complete discussion of the variance of the hydrogen binding energy, charge, density of states and local geometry with strain is presented. The exchange-correlation, electrostatic and kinetic contributions to the binding energy are delineated and their respective contributions are discussed. The mechanism which determines the preferred binding site for each system under strain is shown to be complex.

Formation of Two-Dimensional Copper Selenide on Cu(111) at Very Low Selenium Coverage.

Using scanning tunneling microscopy (STM), we observed that adsorption of Se on Cu(111) produced islands with a (√3×√3)R30° structure at Se coverages far below the structure's ideal coverage of 1/3 monolayer. On the basis of density functional theory (DFT), these islands cannot form due to attractive interactions between chemisorbed Se atoms. DFT showed that incorporating Cu atoms into the √3-Se lattice stabilizes the structure, which provided a plausible explanation for the experimental observations. STM revealed three types of √3 textures. We assigned two of these as two-dimensional layers of strained CuSe, analogous to dense planes of bulk klockmannite (CuSe). Klockmannite has a bulk lattice constant that is 11 % shorter than √3 times the surface lattice constant of Cu(111). This offers a rationale for the differences observed between these textures, for which strain limits the island size or distorts the √3 lattice. STM showed that existing step edges adsorb Se and facet toward ⟨12‾ 1⟩, which is consistent with DFT.

Diffusion of Cu adatoms and dimers on Cu(111) and Ag(111) surfaces

Diffusion of Cu adatoms and dimers on Cu(111) and Ag(111) surfaces is analyzed based on ab initio surface potentials. Single adatom diffusion is compared with dimer diffusion on both surfaces. Surface geometry makes the adatoms jump alternately between two states in the same way in both systems, whereas dimers undergo more complex diffusion process that combines translational and rotational motion. Small difference in the surface lattice constant between Cu and Ag crystals results in a completely different energy landscape for dimer jumps. As an effect the character of diffusion process changes. Homogeneous Cu dimer diffusion is more difficult and dimers rather rotate within single surface cell, whereas diffusion over Ag surface is faster and happens more smoothly. The temperature dependence of diffusion coefficient and its parameters: energy barrier and prefactor is calculated and compared for both surfaces.

Self-organization of S adatoms on Au(111): √3R30° rows at low coverage.

Using scanning tunneling microscopy, we observe an adlayer structure that is dominated by short rows of S atoms, on unreconstructed regions of a Au(111) surface. This structure forms upon adsorption of low S coverage (less than 0.1 monolayer) on a fully reconstructed clean surface at 300 K, then cooling to 5 K for observation. The rows adopt one of three orientations that are rotated by 30° from the close-packed directions of the Au(111) substrate, and adjacent S atoms in the rows are separated by √3 times the surface lattice constant, a. Monte Carlo simulations are performed on lattice-gas models, derived using a limited cluster expansion based on density functional theory energetics. Models which include long-range pairwise interactions (extending to 5a), plus selected trio interactions, successfully reproduce the linear rows of S atoms at reasonable temperatures.

Dysprosium-induced nanowires on Ge(001)

Abstract Dysprosium-induced, self-assembled nanowires on Ge(001) were studied by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). For this, submonolayers of dysprosium were deposited on Ge(001). After subsequent annealing, the surface shows a variety of structures as different kinds of wires with varying thickness, larger islands and surface defects like missing dimers and vacancy rows perpendicular to the Ge(001)-dimer rows. The structures are forming on all intact Ge(001)-terraces. In contrast to dysprosium on Si(001), on Ge(001) no wetting layer is found. In this work we focus on the thinnest nanowires, which have a width of four times the Ge(001)-lattice constant. STM topographies of the unoccupied states show a twofold superstructure on the wires. In contrast, topographies of the occupied states show that the structure of the wires is less regular. They exhibit elevations with distances, which are multiples higher than two of the Ge(001)-surface lattice constant. Density of states mapping reveals a cell structure of the nanowires. Possible explanations for this behaviour are discussed.

Computational studies of atmospherically-relevant chemical reactions in water clusters and on liquid water and ice surfaces.

CONSPECTUS: Reactions on water and ice surfaces and in other aqueous media are ubiquitous in the atmosphere, but the microscopic mechanisms of most of these processes are as yet unknown. This Account examines recent progress in atomistic simulations of such reactions and the insights provided into mechanisms and interpretation of experiments. Illustrative examples are discussed. The main computational approaches employed are classical trajectory simulations using interaction potentials derived from quantum chemical methods. This comprises both ab initio molecular dynamics (AIMD) and semiempirical molecular dynamics (SEMD), the latter referring to semiempirical quantum chemical methods. Presented examples are as follows: (i) Reaction of the (NO(+))(NO3(-)) ion pair with a water cluster to produce the atmospherically important HONO and HNO3. The simulations show that a cluster with four water molecules describes the reaction. This provides a hydrogen-bonding network supporting the transition state. The reaction is triggered by thermal structural fluctuations, and ultrafast changes in atomic partial charges play a key role. This is an example where a reaction in a small cluster can provide a model for a corresponding bulk process. The results support the proposed mechanism for production of HONO by hydrolysis of NO2 (N2O4). (ii) The reactions of gaseous HCl with N2O4 and N2O5 on liquid water surfaces. Ionization of HCl at the water/air interface is followed by nucleophilic attack of Cl(-) on N2O4 or N2O5. Both reactions proceed by an SN2 mechanism. The products are ClNO and ClNO2, precursors of atmospheric atomic chlorine. Because this mechanism cannot result from a cluster too small for HCl ionization, an extended water film model was simulated. The results explain ClNO formation experiments. Predicted ClNO2 formation is less efficient. (iii) Ionization of acids at ice surfaces. No ionization is found on ideal crystalline surfaces, but the process is efficient on isolated defects where it involves formation of H3O(+)-acid anion contact ion pairs. This behavior is found in simulations of a model of the ice quasi-liquid layer corresponding to large defect concentrations in crystalline ice. The results are in accord with experiments. (iv) Ionization of acids on wet quartz. A monolayer of water on hydroxylated silica is ordered even at room temperature, but the surface lattice constant differs significantly from that of crystalline ice. The ionization processes of HCl and H2SO4 are of high yield and occur in a few picoseconds. The results are in accord with experimental spectroscopy. (v) Photochemical reactions on water and ice. These simulations require excited state quantum chemical methods. The electronic absorption spectrum of methyl hydroperoxide adsorbed on a large ice cluster is strongly blue-shifted relative to the isolated molecule. The measured and calculated adsorption band low-frequency tails are in agreement. A simple model of photodynamics assumes prompt electronic relaxation of the excited peroxide due to the ice surface. SEMD simulations support this, with the important finding that the photochemistry takes place mainly on the ground state. In conclusion, dynamics simulations using quantum chemical potentials are a useful tool in atmospheric chemistry of water media, capable of comparison with experiment.

Chemical and mineralogical composition of ilmenite: Effects on physical and surface properties

Some physical and chemical properties of three ilmenite samples: IL-F (purchased from the mineral dealership of A and F Krantz), IL-Q (Qara-aghaj deposit, Iran) and IL-K (Kahnuj deposit, Iran) were studied. The substitution of different amounts of Mg, Mn and V in the crystal structure of ilmenite was identified by microprobe analysis. Different amounts of hemoilmenite exsolution were observed inside all three ilmenite samples using SEM, electron microprobe and X-ray diffraction analysis. Using these analytical techniques, titanite phase was found inside the IL-K ilmenite. The XPS analysis indicated that the contents of Fe3+ are 5.66, 5.33 and 3.72% for IL-F, IL-Q and IL-K, respectively which are in good agreement with the amounts of hemoilmenite exsolution. Mg and V contents in the ilmenite crystal structure have a good negative correlation with lattice constants (LC), unit cell volume, crystallinity index, specific gravity and IEP, while the Mn content correlates positively with these parameters. The IEP determined as 4.2, 5.4 and 6.25 for IL-F, IL-Q and IL-K, respectively, has a negative correlation with Fe3+ ions and ilmenite floatability. In comparison with magnetic susceptibility, the electrical resistivity of ilmenite has a higher correlation with surface properties (IEP and floatability), lattice constants, and unit cell volume.

Computer simulation of epitaxial nucleation of a crystal on a crystalline surface

We present results of computer simulations of crystal nucleation on a crystalline surface, in the Lennard-Jones model. Motivated by the pioneering work of Turnbull and Vonnegut [Ind. Eng. Chem. 44, 1292 (1952)], we investigate the effects of a mismatch between the surface lattice constant and that of the bulk nucleating crystal. We find that the nucleation rate is maximum close to, but not exactly at, zero mismatch. The offset is due to the finite size of the nucleus. In agreement with a number of experiments, we find that even for large mismatches of 10% or more, the formation of the crystal can be epitaxial, meaning that the crystals that nucleate have a fixed orientation with respect to the surface lattice. However, nucleation is not always epitaxial, and loss of epitaxy does affect how the rate varies with mismatch. The surface lattice strongly influences the nucleation rate. We show that the epitaxy observed in our simulations can be predicted using calculations of the potential energy between the surface and the first layer of the nucleating crystal, in the spirit of simple approaches such as that of Hillier and Ward [Phys. Rev. B 54, 14037 (1996)].

Real-time observations of ultra-thin iron oxide film growth on oxygen-deficient YSZ(001)

We use low energy electron microscopy (LEEM) and low energy electron diffraction (LEED) to characterize the initial stages of iron oxide film growth on oxygen-deficient YSZ(001). The films are grown by Fe deposition in a background of 10−6 to 10−5Torr O2. The first layer grows as FeO(111) with four non-equivalent domains arising from two rotational orientations and two stacking sequences. Uniform spreading of 2-D islands is observed by initiating growth at ~1000°C and raising the temperature to 1110–1145°C during Fe deposition. The growth is anisotropic with the fast growth direction depending strongly on both the rotational and stacking domain structure, most likely the result of preferred O2 dissociation at specific island edge configurations. The FeO(111) film has a distinct LEEM-IV spectrum with three well-defined maxima and is easily distinguished from the YSZ(001) substrate. The coarsening of small islands (<10nm diameter) at temperatures above 1160°C rotates the film orientation by 15° with respect to the substrate and reduces the coverage by about one half suggesting a dewetting process. After completion of the first layer, islands with a surface lattice constant corresponding to Fe3O4/γ-Fe2O3 appear with a LEEM-IV fingerprint different from both FeO(111) and the YSZ(001) substrate.

Adsorbate-induced reconstruction by C60on close-packed metal surfaces: Mechanism for different types of reconstruction

Recent studies reveal that reconstruction of close-packed metal surfaces induced by C${}_{60}$ adsorption is the rule rather than the exception. Two types of reconstruction are reported for C${}_{60}$ on different surfaces: (1) C${}_{60}$ sinks into a 7-atom hole, such as on Cu(111); and (2) C${}_{60}$ sits over a 1-atom hole, such as on Ag(111) and Pt(111). An explanation for the preferred reconstruction type for different metals has been lacking. Here, we propose a criterion that predicts which reconstruction type should be expected: Namely, the formation of a 1- or 7-atom hole is determined only by the substrate geometric structure, including the surface lattice constant and the interlayer spacing; remarkably, the reconstruction type appears not to depend on the substrate electronic structure. Our intuitive geometrical explanation is validated by comparative first-principles calculations of the energetics of C${}_{60}$ on Cu(111), Ru(0001), Pt(111), and Ag(111) surfaces, listed here with increasing surface lattice constant. This provides a uniform explanation for the different reconstruction types that are observed experimentally for C${}_{60}$ on different close-packed metal surfaces. Moreover, our results provide a better explanation for the decomposition behavior of C${}_{60}$ on Ru(0001).

Characteristics of charge density waves on the surfaces of quasi-one-dimensional charge-transfer complex layered organic crystals

We performed scanning tunneling microscope (STM) studies to observe the surface electronic structure of the following semiconducting charge-transfer complexes: dipropylamine-tetracyanoquinodimethane [DPA(TCNQ)${}_{2}$], triethylammonium [TEA (TCNQ)${}_{2}$], and $N$-ethylmorpholinium [HEM(TCNQ)${}_{2}$], which all possess the molecularly flat $\mathit{bc}$ surface terminated predominately with the quasi-one-dimensional (1D) TCNQ${}^{\ensuremath{-}0.5}$ anion chains or the corresponding DPA${}^{+}$, TEA${}^{+}$, and HEM${}^{+}$ cations contributing a lesser fraction. On the $\mathit{bc}$ surfaces terminated with TCNQ${}^{\ensuremath{-}0.5}$ anions for all crystals, the $4{k}_{F}$[DPA(TCNQ)${}_{2}$] or $2{k}_{F}$ [TEA(TCNQ)${}_{2}$ and HEM(TCNQ)${}_{2}$] charge density wave (CDW) driven by the spin-Peierls or Peierls instability for the quasi-1D TCNQ${}^{\ensuremath{-}0.5}$ chains, respectively, which occur already within the bulk, were probed by use of STM on the crystal surfaces. In that the surface lattice constants obtained from CDW are in good agreement with the bulk ones obtained by x-ray diffraction, such CDWs are commensurate. This modulation of the electric structure is so distinct that the geometric corrugation of individual TCNQ anions was hardly traced during our STM measurements. On the $\mathit{bc}$ surfaces terminated with DPA${}^{+}$, TEA${}^{+}$, and HEM${}^{+}$ cations, the STM images with submolecular resolution represent the geometric corrugation of individual molecular cations, and the surface lattice constants obtained deviate greatly from that of the x-ray crystal structures, indicating self-reassembling of the surface cations.

Improved lattice constants, surface energies, and CO desorption energies from a semilocal density functional

Using the revised Tao-Perdew-Staroverov-Scuseria (revTPSS) metageneralized gradient approximation, a computationally efficient semilocal functional, we studied the desorption energies of the molecule CO on the (111) surfaces of transition metals as well as the surface energies and lattice constants of the underlying transition metals. Due to its ability to distinguish single-orbital regions from regions of high orbital overlap, revTPSS improves all three properties over the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation. No generalized gradient approximation matches this performance, which has been regarded as unreachable by semilocal approximations.

Electronic structure of germanium nanocrystals core and (001)-(1×1) oxidised surface

The electronic structure of (001)-(1×1) oxidised surface and inner core of germanium nanocrystals (1.5–2.3 nm in diameter) is studied separately using the self-consistent Hartree–Fock method coupled with the large unit cell (LUC) method. The separate evaluation of surface and core electronic structure leads to the exact discrimination between nanocrystal surface and core properties. The results show that the lattice constant and the energy gap of the core part decrease as nanocrystals increase in size in agreement with previous studies. The small differences between the core and the surface lattice constant reflects good adherence of the oxide layer. The nanocrystal oxidised surface is characterised by a smaller gap and a wider valence band than the core. This result is owing to the splitting of the highly degenerate core states as they approach the surface in addition to the newly added levels of oxygen surface atoms. The present two gap results resolved the ambiguity in different gap and photoluminescence results of Ge nanocrystals. The surface layers have oscillatory negative and positive charges that diminish as they reach the inner fourth germanium layer. LUC method predicts nearly direct transitions in small germanium nanocrystals that explain their suitability for light-emitting applications.

Local Density of States of Partially Oxidized Ag(110) Surfaces Observed Using Scanning Tunneling Microscope Light-Emission Spectroscopy

We have observed the scanning tunneling microscope (STM) light-emission spectra of partially oxidized Ag(110) surfaces with atomic spatial resolution. The STM image visualized Ag–O chains along the [001] direction as light rows, separated by a distance n ×a from adjacent light rows. Here, n is an integer between 2 and 7, and a is the surface lattice constant of Ag in the [110] direction. The STM light-emission spectra were measured for tip locations over the light rows and the midpoints of the darkly imaged areas sandwiched by two adjacent light rows. While the spectral shapes of the STM light-emissions were identical, their amplitudes showed atomic-site dependence, i.e., the STM light-emission intensities showed atomic-site dependence without any spectral differences. The STM light-emission intensities at the midpoints of the dark areas increased with n. The STM light-emission intensities of the light rows were independent of the widths of the adjacent dark areas. The experimental results were analyzed by the dielectric theory of STM light-emission, and it was found that the observed atomic-site dependence is caused by electronic states localized along the light rows. These states are identified as the py and pz surface bands induced by adsorbed oxygen.

In situ Video-STM studies of sulfate dynamics on Au(1 1 1)

In situ video-rate scanning tunneling microscopy studies of the anion adlayer on Au(1 1 1) electrodes in sulfate-containing electrolytes are presented that provide direct data on the structure and dynamics of the potential-induced phase transition between the disordered adlayer and the ordered (√3 × √7) structure as well as of local transient structures in the disordered adsorbate phase. Specifically, strong correlations between adsorbates spaced at distances of √3 Au surface lattice constants along the [ 1 1 2 ¯ ] directions of the metal substrate are observed, supporting the formation of a 1D (bi)sulfate network via hydrogen-bonding along this direction. This network structure is in accordance with the rather slow adsorbate dynamics with residence times in the range of milliseconds or larger.